5,867 research outputs found
Magnetic Moment Formation in Quantum Point Contacts
We study the formation of local magnetic moments in quantum point contacts.
Using a Hubbard-like model to describe point contacts formed in a two
dimensional system, we calculate the magnetic moment using the unrestricted
Hartree approximation. We analyze different type of potentials to define the
point contact, for a simple square potential we calculate a phase diagram in
the parameter space (Coulomb repulsion - gate voltage). We also present an
analytical calculation of the susceptibility to give explicit conditions for
the occurrence of a local moment, we present a simple scaling argument to
analyze how the stability of the magnetic moment depends on the point contact
dimensions.Comment: 7 pages, 2 figure
Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts
The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched
GaAs/GaAlAs quantum point contacts, by measuring the differential conductance
as a function of source-drain and gate bias as well as a function of
temperature. We investigate in detail how, for a given gate voltage, the
differential conductance depends on the finite bias voltage and find a
so-called self-gating effect, which we correct for. The 0.7 anomaly at zero
bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at
finite bias. Varying the gate voltage the transition between the 1.0 and the
0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate
voltage dependent energy difference . This energy difference is
compared with the activation temperature T_a extracted from the experimentally
observed activated behavior of the 0.7 anomaly at low bias. We find \Delta =
k_B T_a which lends support to the idea that the conductance anomaly is due to
transmission through two conduction channels, of which the one with its subband
edge \Delta below the chemical potential becomes thermally depopulated as the
temperature is increased.Comment: 9 pages (RevTex) with 9 figures (some in low resolution
Quantum Interaction : the Construction of Quantum Field defined as a Bilinear Form
We construct the solution of the quantum wave equation
as a bilinear form which can
be expanded over Wick polynomials of the free -field, and where
is defined as the normal ordered product with
respect to the free -field. The constructed solution is correctly defined
as a bilinear form on , where is a
dense linear subspace in the Fock space of the free -field. On
the diagonal Wick symbol of this bilinear form
satisfies the nonlinear classical wave equation.Comment: 32 pages, LaTe
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
Observation of Quantum Asymmetry in an Aharonov-Bohm Ring
We have investigated the Aharonov-Bohm effect in a one-dimensional
GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of
these systems are for the first time systematically studied as a function of
density. We observe phase-shifts of in the magnetoconductance
oscillations, and halving of the fundamental period, as the density is
varied. Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.Comment: 4 pages RevTex including 3 figures, submitted to Phys. Rev.
Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B
Aims. The aim of this paper is to study deuterated water in the solar-type
protostars NGC1333 IRAS4A and IRAS4B, to compare their HDO abundance
distribution with other star-forming regions, and to constrain their HDO/H2O
ratios. Methods. Using the Herschel/HIFI instrument as well as ground-based
telescopes, we observed several HDO lines covering a large excitation range
(Eup/k=22-168 K) towards these protostars and an outflow position. Non-LTE
radiative transfer codes were then used to determine the HDO abundance profiles
in these sources. Results. The HDO fundamental line profiles show a very broad
component, tracing the molecular outflows, in addition to a narrower emission
component and a narrow absorbing component. In the protostellar envelope of
NGC1333 IRAS4A, the HDO inner (T>100 K) and outer (T<100 K) abundances with
respect to H2 are estimated at 7.5x10^{-9} and 1.2x10^{-11}, respectively,
whereas, in NGC1333 IRAS4B, they are 1.0x10^{-8} and 1.2x10^{-10},
respectively. Similarly to the low-mass protostar IRAS16293-2422, an absorbing
outer layer with an enhanced abundance of deuterated water is required to
reproduce the absorbing components seen in the fundamental lines at 465 and 894
GHz in both sources. This water-rich layer is probably extended enough to
encompass the two sources as well as parts of the outflows. In the outflows
emanating from NGC1333 IRAS4A, the HDO column density is estimated at about
(2-4)x10^{13} cm^{-2}, leading to an abundance of about (0.7-1.9)x10^{-9}. An
HDO/H2O ratio between 7x10^{-4} and 9x10^{-2} is derived in the outflows. In
the warm inner regions of these two sources, we estimate the HDO/H2O ratios at
about 1x10^{-4}-4x10^{-3}. This ratio seems higher (a few %) in the cold
envelope of IRAS4A, whose possible origin is discussed in relation to formation
processes of HDO and H2O.Comment: 16 pages, 13 figure
Water distribution in shocked regions of the NGC1333-IRAS4A protostellar outflow
We present the study of the H2O spatial distribution at two bright shocked
regions along IRAS4A, one of the strongest H2O emitters among the Class 0
outflows. We obtained Herschel-PACS maps of the IRAS4A outflow and HIFI
observations of two shocked positions. The largest HIFI beam of 38 arcsec at
557 GHz was mapped in several key water lines with different upper energy
levels, to reveal possible spatial variations of the line profiles. We detect
four H2O lines and CO (16-15) at the two selected positions. In addition,
transitions from related outflow and envelope tracers are detected. Different
gas components associated with the shock are identified in the H2O emission. In
particular, at the head of the red lobe of the outflow, two distinct gas
components with different excitation conditions are distinguished in the HIFI
emission maps: a compact component, detected in the ground-state water lines,
and a more extended one. Assuming that these two components correspond to two
different temperature components observed in previous H2O and CO studies, the
excitation analysis of the H2O emission suggests that the compact (about 3
arcsec) component is associated with a hot (T~1000 K) gas with densities
~(1-4)x10^5 cm^{-3}, whereas the extended one (10-17 arcsec) traces a warm
(T~300-500 K) and dense gas (~(3-5)x10^7 cm^{-3}). Finally, using the CO
(16-15) emission observed at R2, we estimate the H2O/H2 abundance of the warm
and hot components to be (7-10)x10^{-7} and (3-7)x10^{-5}. Our data allowed us,
for the first time, to resolve spatially the two temperature components
previously observed with HIFI and PACS. We propose that the compact hot
component may be associated with the jet that impacts the surrounding material,
whereas the warm, dense, and extended component originates from the compression
of the ambient gas by the propagating flow.Comment: 13 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
Perturbative Hamiltonian constraints for higher order theories
We present a method for constructing a consistent low energy canonical
formalism for higher order time-derivative theories, extending the Dirac method
to include perturbative Hamiltonian constraints. We apply it to two
paradigmatic examples: the Pais-Uhlenbeck oscillator and the Bernard-Duncan
scalar field. We also compare the results, both at the classical and quantum
level, with the ones corresponding to a direct perturbative construction
applied to the exact higher order theory. This comparison highligths the
soundness of the present formalism.Comment: 26 pages, 4 figures; review section shortened and appendices change
Accelerated high-cycle phase field fatigue predictions
Phase field fracture models have seen widespread application in the last
decade. Among these applications, its use to model the evolution of fatigue
cracks has attracted particular interest, as fatigue damage behaviour can be
predicted for arbitrary loading histories, dimensions and complexity of the
cracking phenomena at play. However, while cycle-by-cycle calculations are
remarkably flexible, they are also computationally expensive, hindering the
applicability of phase field fatigue models for technologically-relevant
problems. In this work, a computational framework for accelerating phase field
fatigue calculations is presented. Two novel acceleration strategies are
proposed, which can be used in tandem and together with other existing
acceleration schemes from the literature. The computational performance of the
proposed methods is documented through a series of 2D and 3D boundary value
problems, highlighting the robustness and efficiency of the framework even in
complex fatigue problems. The observed reduction in computation time using both
of the proposed methods in tandem is shown to reach a speed-up factor of 32,
with a scaling trend enabling even greater reductions in problems with more
load cycles
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